A major concern in the plasma processing of materials is the spatial uniformity of the process. Process uniformity depends strongly on the spatial distribution of both charged and neutral particle densities and velocities as well as potentials. Process uniformity is a key requirement to enable large semiconductor wafers or flat panels to be processed by plasma.
Electrical probes have long been used with plasmas to measure electron density, Electron Energy Distribution Function (EEDF), potentials, and electromagnetic fields. See, e.g., F. F. Chen, in Plasma Diagnostic Techniques, edited by R. H. Huddlestone and S. L. Leonard, Academic Press, New York, 1965, pp. 113-200, and L. Schott, in Plasma Diagnostics, edited by W. Lochte-Holtgreven, North-Holland Publishers, Amsterdam, 1968, pp. 668-731. Mach probes are electrical probes that have the capability of measuring ion flow velocities. See, e.g., I. H. Hutchinson, Principles of Plasma Diagnostics, Cambridge University Press, New York, 1987.
Optical Emission Spectroscopy (OES) has been extensively used in plasma processing to obtain optical emission information that is integrated along the line-of-sight. To obtain spatial resolution, Abel inversion of a multi-direction OES is most frequently used, assuming cylindrical symmetry of the plasma. In conventional actinometry, a known quantity of an inert gas is added to the reactive gas and the distribution of radicals is obtained by analyzing the ratio of light emitted from the radicals and the reference gas, with argon typically being used with fluorine and xenon with chlorine. The excitation rate ratio between the reference gas and the radical species depends strongly on the local EEDF. In conventional actinometry, the emitted light is received from a volume which typically extends across the chamber containing the plasma so that the measurements of the emitted light that are obtained provide only the average emission over the entire volume within the chamber. This limits the accuracy of the actinometry measurements since the EEDF is typically not uniform through the whole volume over which the light emissions are detected.